I'm just getting back to playing on keyboard. I'm trying to play "Ode to Joy" from the piano sheet but I have a basic question.

At the beginning of some staffs, on the right of the treble clef I can see a key signature with single sharp (#) sign on the upper-F position. From what I understand, that means that I have raise each "F" note half a tone each time I play it. That seems reasonable but in the staffs starting with this key signature I can't find any "F" notes! So what is that sharp symbol for?

Maybe I should modify (play differently) chords too? If so, in what way and which of the chords (or maybe all of them)?

I have to give this question the "Most bizzare music question I've ever seen award." While I understand your confusion, imagine asking the same question about ledger lines. Just because some piece never gets down to, say, E above middle C (treble clef) doesn't mean you'd want the lowest ledger line to be left out.
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Carl WitthoftJun 20 '13 at 11:38

@CarlWitthoft Please, note that I wasn't playing music for a longer time (actually more than 10 years) and I forgot hell lot theoretical stuff :-(. So it may seem obvious or bizzare for others but it was a doubt for me when I was trying to play this piece and wondering how should it influence the way I play it. Anyway, thanks for lowest ledger line metaphor :).
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Piotr SobczykJun 20 '13 at 11:53

@CarlWitthoft - nice analogy, but ledger lines are the little lines above or below the staff which indicate a note too high or too low to fit into the staff itself. Unless told to, some computer written music won't have a key sig., which can be very confusing,as one has to guess. This tune COULD have been in E minor, from the key sig.
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TimJun 20 '13 at 12:06

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Sorry, pre-coffee brain meant to type 'stave line' but missed. And of course the piece could have been in e, or in D-Mixolydian :-). BTW I wouldn't go blaming the computer for failure to provide a key signature--it's the fault of the programmer.
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Carl WitthoftJun 20 '13 at 12:14

Could you please elaborate a little more about "Even if there are no F notes, it's important to know what key you're in"? If I don't have to modify anything then why is it important for me? How should it influence the way I play this melody?
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Piotr SobczykJun 19 '13 at 18:09

7

Because that F# is telling you that the piece is in G major, so you can emphasize the G's if you want to, or if you decide to stray a little from what's written and do some chord inversions and/or substitutions, you know what key you're in. Also, knowing what key you're in will help you read the music because you don't have to read every note, you can just recognize patterns and determine what chord to play from that.
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ChochosJun 19 '13 at 20:43

Without the key signature is looks confusing and wrong to musicians who are used to reading music in context, instead of just treating sheet music as a "play by numbers" game. It looks like C major, but the notes are mostly confined to the GABCD range, and the theme ends on G.

Indeed, an F note will indeed occur in harmonizations of the theme. For instance the V chord in G major is a D, which contains the F# (D F# A).

Some musicians can harmonize without having the notes spelled out in the sheet music; it helps them to know from the outset that the key is G. That key signature clue instantly connects in their minds to a scale pattern on the instrument, from which chords emerge.

As Chochos and Kaz said, it lets you know that the key is G. Kaz described some things that knowing the key will let you do. Here are some more:

Since the song is in the key of G, the note G will sound more consonant than any other note, followed by a D (the fifth of the scale). These notes will sound more peaceful than any other note, and most phrases will end on one of them.

Similarly, a G chord will sound more resolved than any other chord. In fact, you can expect with almost full certainty that the G chord will appear more frequently than any other chord. (And if it doesn't, that lets you know something about the piece you're playing.)

The least consonant notes will be F#, G#, and Db (the accidentals right next to the G and D). The least consonant chords are F# and Db. These notes and chords will almost always sound harsh, tense, or at least colorful.

Knowing this much, you can start to understand where a song gets its emotion from if you analyze it. A jazz piece written in G at 180 bpm will sound frantic if it has lots of F#s and Dbs. A string quartet piece in G written at 90 BPM with only G and D chords will sound peaceful.

A key signature can be interpreted on (at least:) two levels. On simple level it signals which notes to raise/lower by a halftone. You correctly assume, that it could be left out in your specific case. It is not done however, since this is a conflict with the following: on sophisticated level it signals the key of the piece. This shows, that the first and last notes or chords are likely in that harmony, what harmonies to add if improvising or adding another instrument or voice. Recognition of zero to six sharps or flats is difficult enough, a decoding of something like "e flat major, but second flat omitted" is not helpful. This does not restrict pieces in any way, since the accidentals can be modified, where needed.

Actually, one could argue in favor of using a separate set of key signatures for harmonic minor (e.g. C minor would have Eb and Ab but not Bb; D minor would have Bb and C#). That's not common practice (the convention is to write major-ish pieces in the key signature for the corresponding Ionian mode, and minor-ish pieces in the key signature for the corresponding Aeolian mode, and put in whatever accidentals are made necessary by such usage.
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supercatApr 25 '14 at 19:24

Start on C and play through all the white keys - C, D, etc, ending on C. The pattern of sounds you just heard is called a "major scale". The key signature of C has no sharps or flats because to play that major C scale, you don't need any.

The key signature of G major is telling you "if you want to play that same 'major scale tune' in G, start from G and do the same thing EXCEPT with a raised F." Play the all the white notes starting from G and it won't sound like a major scale. Substitute F# for F and it will.

If you don't play any Fs, it won't matter, but if you wanted to improvise with notes that sound "good" in the key of G, you'd need to sharp the Fs.

The piece you are trying to play is in G Major. More than merely indicating an alteration to one or more notes, the key signature gives you the exact notion of the harmonic field you are working into. Should you play accompanying chords or do some polyphony (voice leading, for instance) and the key signature would give you the exact tones of the scale that are available for you to use to create such an overall sonority. It is important, for that matter, that you are aware that the F note is always sharp when you are playing diatonic chords in the G Major key.

It is always important to think of music not being limited to any specific context (melodic, rhythmic or harmonic, among many others). Soon you are going to (I hope) feel the need to fill the harmonic gaps with more and more complex layers of tones. Knowing beforehand the sound materials that are available for each tonality is very important, as tonality is a narrowing abstraction with its own systematics and specific rules.

Even if you don't play the notes it's handy to know what key you're in, for the purpose of improvisation, your own chords and other little tweaks specific to each performer.

Knowing your Mode
Let's say you're in G but you never play F#. The difference between displaying the key and not to me is that with the key signature displayed you're in G major. Without it you're in G Mixolydian.

This would have an effect if you added chords that aren't written or added little turns/additions to the melody, because you'd be using the F natural as if you're in G mixolydian rather than the F Sharp as if you're in G Major/Ionian.

If you are video conferencing with an attorney in a suit and would be asked afterwards to make a sketch of the attorney, it would not make sense to draw the attorney without any pants on even if the recording did not actually show whether the attorney might have had a clothing accidental.

In the context of G major, nothing but an F# makes harmonic sense, whether or not you actually played it.

As long as you are keeping strictly to the script, it will not actually be possible to tell the difference from the execution. But it may make a difference about how you think about the thing and how it could develop.